Four main fan deltas, Ohtani river fan delta, Hira river fan delta, Yamune river fan delta and Takigawa fan deltas, are developed along the eastern margin of Hira Mountains, western coast of Lake Biwa.Subaerial part of the each fan deltas are geomorphogically subdivided into upper and lower fan segments. The upper fan shows conical shape and comprises dominantly boulder and cobble beds which deposited by debris flow and sheet flow processes. The lower fan consists of elongated lobes and inter lobe depressions. The lobe deposits were dominantly deposited by stream flow process.These fans are submerged directly into the lake water forming delta slopes. Subaqueous investigation using ROV (Remotely Operate Vehicle) reveals that the underwater part of these fan deltas form steep-face delta slopes. The deposits on these delta slopes show a downslope decrease in clast size. Some delta-slope deposits exhibit accumulations of coarse debris composed of cobbles and boulders with chaotic sand-to silt-matrix indicating a coastal landsliding on the delta slope.